The present invention relates to a television receiver for simultaneously displaying a plurality of programs on a single screen of a television receiver.
A demand of effective use of a television screen drives one to recently develope a television receiver of the type in which a small size picture (hereinafter referred to as a sub-picture) is displayed in a large size picture (hereinafter referred to as a main picture), those pictures being assigned for different programs, respectively, as disclosed in Japanese Laid-open Patent Publication No. 2419/74, DT-OS No. 2,239,593, and electronics journal "Nikkei Electronics" Dec. 26, 1977, p.p. 127 to 134 published in Japan. This type of the television receiver is generally called a picture-in-picture (PinP) television. The PinP television will be described in brief referring to FIGS. 1 to 3.
In FIG. 1 illustrating a concept of the PinP TV, reference numeral 1 designates a TV receiver; 2 a cathode ray tube (hereinafter referred to as a CRT); 3 a main picture portion; 4 a sub-picture portion having a picture different from that in the main picture portion. The main picture and sub-picture portions are so arranged as to individually select different channels for displaying pictures of the selected channels.
FIG. 2 diagramatically illustrates an example how the sub-picture is inserted into the main picture. In the figure, reference character I designates the sub-picture before its size is reduced and II the main picture with the sub-picture reduced. It is assumed now that a picture reduction rate of the sub-picture is 1/3 for both the vertical and horizontal sides of the picture. Here, the picture reduction rate is defined as a ratio of a scanning period after it is reduced and a scanning period of the original signal. The scanning lines are extracted from the sub-picture before it is reduced, one for three scanning lines. The horizontal period is time-reduced into 1/3. Then, it is synchronized with the main picture. Finally, thus processed picture is inserted into the main picture. Scanning lines 1 to 3 are exemplary lines of those of the picture before and after it is reduced.
FIG. 3 is a circuit construction of part of a conventional PinP TV receiver, which relates to the present invention. In the figure, reference numeral 11 designates an antenna; 12 a sub-picture insertion circuit; 13 a video signal processing circuit; 14 a display device using a CRT; 21 and 31 tuners for the main picture and sub-picture; 22 and 32 IF video detecting circuits for the main picture and sub-picture, respectively.
A signal for the sub-picture is stored into a memory 41 under control of a synchronizing (hereinafter abbreviated as a sync) signal obtained from a sync separation circuit 33 and a clock signal from a write clock generating circuit 42. Reference numeral 43 designates a read clock generating circuit. The sub-picture signal from the memory 41 is read out under control of the signal from the sync separation circuit 23 for each scanning line, and then is inserted into a main picture signal. The example is for a case where both the main picture and sub-picture are of the monochrome type.
When both the pictures are of the color type, the sub-picture signal is read out from the memory, and the read out one and the main picture signal are composed. The reference level of the color difference signal changes due to temperature drift and aging of the circuit components. As a result, the white balance by the main picture signal is not coincident with that by the sub-picture signal.